1. Field of the Invention
The present invention concerns a method of controlling the recovery of landfill gas from a sanitary landfill by measuring the temperature of the gas and by adapting the gas withdrawal thereby as well as apparatus for performing the method.
2. Description of the Prior Art
In a landfill of domestic refuse and specific types of industrial waste, so-called landfill gas is formed after some time in consequence of the decomposition of the organic contents of the refuse. The landfill gas commonly consists of 50-60% methane, about 40% of carbon dioxide and 2-10% of nitrogen. The landfill gas is formed under anaerobic conditions, i.e. absence of air.
The anaerobic decomposition of the organic material in the waste to landfill gas is a slightly exothermic process, i.e. a very small quantity of heat is developed. If oxygen is supplied to the refuse, for example by penetration of air, an aerobic decomposition of the organic material to mainly carbon dioxide and water occurs. This aerobic process is highly exothermic, i.e. large quantities of heat are developed in connection with the decomposition process.
Landfill gas is generally recovered from a sanitary landfill by perforated tubes being urged downwardly from the surface into the landfill. In order to prevent air from mixing with the landfill gas the upper 80-160 inches of the tube must not have perforations. The gas is commonly supplied from such type of a so-called gas well through a conduit to a control chamber. In this chamber the gas conduit coming from the gas well is provided with a valve for controlling the gas flow and a gas stoplet for analysis samples. Gas conduits are connected to a control chamber of this type from a plurality of gas wells, normally 2-6 of such wells. These conduits are thereafter combined in the control chamber so as to form a manifold through which the gas is drawn to a fan or a compressor. This equipment then urges the gas on to the place of utilization which may be a furnace or boiler, a gas engine, etc.
The gas well and the conduit system are subjected to negative pressure up to the fan. If the quantity of gas which is withdrawn from the gas well is greater than the quantity of gas which is formed in the sanitary landfill around the gas well, air will be drawn into the landfill. The decomposition process in the landfill will then partly be aerobic, which i.a. causes the methane content of the gas to decrease and the nitrogen content of the gas to increase. The oxygen content normally only increases slightly, as the major part of the oxygen included in the air being drawn in will be consumed in the aerobic decomposition process in the refuse. As has been mentioned earlier a substantial amount of heat is developed in the aerobic decomposition process, and in drawing in air into the sanitary landfill the temperature of the recovered gas will consequently increase.
In order to achieve high economy in recovering gas from the sanitary landfill, there is a desire to withdraw as much gas as possible from each gas well without incurring the risk of having air drawn into the landfill. Up to now this problem has been solved by regularly checking the methane content of each gas well. As this check normally is carried out at intervals of 4-6 weeks, it becomes very demanding in terms of work and is also expensive. As the result of the lengthy intervals between the tests, air may be drawn into the landfill during a relatively long period before this is discovered. A gas having a low methane content will then be the result, but the greatest danger is that the anaerobic decomposition process is slowed down, and it may take a long time to make this operate satisfactorily again.
On the market there are automatic gas analyzers which can be interconnected to each gas well via gas conduits and can analyze outgoing gas continuously. Equipment of this type is described in European Patent EP 0 182 143, according to which landfill gas is drawn from each gas well to an automatic gas analyzer in which the content of gas of any one of the components O.sub.2, N.sub.2, CO.sub.2 or CH.sub.4 is determined. This measured value will then control the withdrawal of gas from the respective gas well. A control system of this type is of course extremely expensive to set up, and furthermore it requires qualified maintenance of specific system components.
Another way of avoiding having air drawn into the sanitary landfill is described in German Patent Specification DE 27 19 889. According to this publication the pressure inside the sanitary landfill is measured at different gas withdrawal flows before the landfill gas system is set into routine production. That flow is then determined at which the pressure in the landfill in the immediate vicinity of the gas well is the same as the atmospheric pressure. This is the maximum permissible gas withdrawal. After the system has been put into routine production this maximum gas withdrawal must not be exceeded.
The manner of achieving the greatest possible quantity of gas from refuse is described in J. Chem. Tech. Biotechnol., 1980, Volume 30, pages 458-465, wherein the microbiological conditions for anaerobic decomposition with mesophilic microorganisms are described. The author thereafter explains how the moisture content and the temperature affect the reaction rate and finally which external changes can be carried out in the form of insulating the landfill and increasing the moisture content of the refuse so as to improve the gas production.